1. Field of the Invention
The present invention relates to an adaptive beamforming method, and more particularly to an adaptive beamforming method of an indirect type array antenna in a communication system.
2. Background of the Related Art
Conventionally, a beamforming method of an array antenna in a direct sequence code division multiple access (DS-CDMA) system includes a method using only pilot channel information, or a blind algorithm method using only traffic channel information without the pilot channel information.
With respect to the method using only the pilot channel information, there exists a least mean square (hereinafter referred to as LMS) algorithm, and a recursive least square (hereinafter referred to as RLS) algorithm.
With respect to the blind algorithm method, which does not use reference signals (i.e., training signals) known to both a sending end and a receiving end, a constant modulus algorithm (CMA) method, and a 2-dimensional rake combiner method exist.
The principle of forming an adaptive beam of an array antenna according to the LMS algorithm will now be described.
The LMS algorithm is a kind of an adaptive algorithm using a data channel for transmitting actual user information and a channel for transmitting the reference signals (training signals) known to both the sending end and the receiving end.
Since the LMS algorithm uses the reference signals, filter coefficients can be stably updated. Also, since its evaluation function is convex, the convergence to a global minimum values is guaranteed. Also, the LMS algorithm can be implemented through a simple hardware structure, thus simplifying its circuit construction.
The LMS algorithm has been widely used in communication systems which can use the reference signals. The LMS algorithm as described above forms an adaptive beam of the array antenna using a dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH) in reverse dedicated channels.
The slot structures of the reverse channels in a related art system will be described with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating the slot structures of the reverse DPDCH and the reverse DPCCH in the related art system.
Referring to FIG. 1, of the reverse channels, the dedicated channel is used to transfer the user information and the control information from a mobile station (not illustrated) to a base station (not illustrated).
The reverse dedicated channel includes the DPDCH 1 for transmitting user data, and the DPCCH 2 for transmitting pilot information, transmit power control information, and transport format indicator information.
The information per slot transmitted through the DPCCH 2 of the reverse dedicated channel in the system are the pilot information, the transmit power control (TPC) information, and the transport format indicator (TFI) information. The pilot information is used for channel estimation and adaptive beamforming. The TPC is used for open loop power control. The TFI is used for transmitting transport formats in the unit of 16 slots.
FIG. 2 is a block diagram illustrating the construction of the related beamforming apparatus of an array antenna.
Referring to FIG. 2, the signals received through respective antennas (ANT despread through a respective demodulation process through one of corresponding demodulating parts 10, 11, . . . 12, and down-sampling process through one of corresponding downlink sampling and despreading parts 20, 21, . . . 22. Thereafter, weighted values of a beamforming part 30 are updated every moment.
Here, it is assumed that K transmit signals and L antennas exist in the system, and the adaptive beam is formed after the despreading. Also, the adaptive beamforming prior to the despreading can be easily implemented by substituting spreading codes into an equation mentioned below.
If weight vectors of the beamforming part 30 with respect to a first transmitted signal are represented as w1=[w1,1 w2,1 . . . wL,1]xcfx84, the evaluation function Cp(w1[n]) in the LMS algorithm is defined by the following equation (1).
Cp(w1[n])=E[|{overscore (A1+L )}{overscore (s1+L )}[n]xe2x88x92wH1[n]{overscore (P1+L )}[n]|2]xe2x80x83xe2x80x83Eq. (1)
Here, {overscore (A1+L )} is the size of the pilot channel signal of the first transmitted signal, {overscore (s1+L )}[n] is the n-th signal known to the receiving end in advance, and {overscore (P1+L )} is the pilot signal vector despreading through the channel, which is defined by the following equation (2).
{overscore (P1+L )}[n]=[{overscore (P1,1+L )}{overscore (P2,1+L )} . . . {overscore (PL,1+L )}]xcfx84xe2x80x83xe2x80x83Eq. (2)
The LMS algorithm according to equations (1) and (2) can repeatedly obtain the optimum coefficient defined as w1[n] by the following equation (3) using the pilot signal known to the receiving end.
w1[n]=w1[nxe2x88x921]+xcexcpxcex5LMS[n]{overscore (p1+L )}[n]xe2x80x83xe2x80x83Eq. (3)
Here, * denotes a conjugate complex number, and xcex5LMS is defined by the following equation (4).
xcex5LMS={overscore (A1+L )}{overscore (s1+L )}[n]xe2x88x92w1H[nxe2x88x921]{overscore (p1+L )}[n]xe2x80x83xe2x80x83Eq. (4)
Since in the DPDCH 1, only the traffic signal, i.e., the data signal, exists, but the reference signal known to the receiving end in advance does not exist, tentative decision values are obtained using a decision dedicated (DD) algorithm. Here, LMS-DD means obtaining the tentative decision values and using the LMS algorithm.
The evaluation function of the LMS-DD is given by the following equation (5).
Cr(w1[n])=E[|A{overscore (1+L s1+L )}[n]ŝ1[n]xe2x88x92w1H[n]{overscore (r1+L )}[n]|2]xe2x80x83xe2x80x83Eq. (5)
Here, A1 denotes the traffic signal size and ŝ1 denotes the tentative decision value needed to use the LMS algorithm. This value can be represented by the following equation (6).
ŝ1[n]=dec{w1H[n]{overscore (r1+L )}[n]}xe2x80x83xe2x80x83Eq. (6)
In equation (6), dec{} denotes a detection function. In order to guarantee the reliability of the tentative decision value, the coefficient updating is not performed with respect to the signal which is below the threshold value and has a severe fading effect (a case of a bypass mode).
Accordingly, the despreading signal vector {overscore (r1+L )}[n] which is received by the receiver and the value of which is not known in advance is defined by the following equation.
{overscore (r1+L )}[n]=[{overscore (r1,1+L )}{overscore (r2,1+L )} . . . {overscore (rL,1+L )}]xcfx84xe2x80x83xe2x80x83Eq. (7)
Also, the filter coefficient in the DD-LMS algorithm is updated according to the following equation (8).
w1[n])=w1[nxe2x88x921]+xcexcrxcex5*DD[n]{overscore (r1+L )}[n]xe2x80x83xe2x80x83Eq. (8)
Here, xcex5DD is defined by the following equation (9).
xcex5DD=A1{overscore (s)}1[n]Ŝ1[n]xe2x88x92w1H[n]{overscore (r)}1[n]xe2x80x83xe2x80x83Eq. (9)
As described above, since the DPCCH 2 of the reverse dedicated channels is not constructed to purely support the pilot channel, the TPC and the TFI parts, where the pilot information cannot be used, are not used for the adaptive beamforming. Specifically, the system as described above does not have any technique for implementing the above-described adaptive beamforming method.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
An object of the present invention is to provide an adaptive beamforming apparatus and method of an array antenna in a communication system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide an adaptive beamforming apparatus and method of an array antenna in a communication system which can increase the reverse system capacity of the system and wide-band CDMA next-generation mobile communication system.
Another object of the present invention is to provide an adaptive beamforming method of an array antenna in a communication system which can substantially remove an interference effect and thus maximize an antenna diversity effect.
To achieve at least the above objects in whole or in parts, there is provided an adaptive beamforming method that uses the combination of the Decision Dedicated Least Mean Square (DD-LMS) algorithm using the tentative symbol decision value of the traffic signal and the LMS algorithm using the pilot symbol when forming the adaptive beam of the array antenna with the despreading signal. In other words, the tentative symbol decision value is obtained by processing the traffic signal vector first. Thereafter, it is assumed that the tentative symbol decision value is the transmitted symbol value, and the final updated value of the coefficient is obtained by performing at least once the LMS algorithm with respect to the tentative symbol decision value and the pilot signal vector. Last, an adaptive beam corresponding to the final symbol decision value is formed using the traffic signal vector and the final updated value of the coefficient.
To further achieve the above-described objects of the present invention in a whole or in parts, there is provided an adaptive beamforming apparatus of an array antenna, including a first beamforming element, coupled to receive a first signal vector and to form a first adaptive beam, a detection function element, coupled to receive the first adaptive beam and to form a tentative symbol decision value, a first least mean square (LMS) element, coupled to receive at least one of a second signal vector, a known vector, and a feedback signal to generate a first updated coefficient value, a second LMS element, coupled to receive at least one of the first signal vector, the tentative symbol decision value, and the first updated coefficient value to form a second updated coefficient value, and a second beamforming element, coupled to receive the despreading traffic signal vector and the second updated coefficient value and to form a second adaptive beam.
To further achieve the above-described objects of the present invention in a whole or in parts, there is provided an adaptive beamforming apparatus of an array antenna, including a first beamforming element coupled to receive at least one of a first signal vector and a feedback signal to form a first adaptive beam, a decision element coupled to receive the first adaptive beam and output a tentative symbol decision value, a least mean square (LMS) element coupled to receive at least one of the first signal vector, the tentative symbol decision value, a known vector, a second signal vector, and the feedback signal to form an updated coefficient value, and a second beamforming element coupled to receive the first signal vector and the updated coefficient value to generate a final symbol decision value as an adaptive beam.
To further achieve the above-described objects of the present invention in a whole or in parts, there is provided an adaptive beamforming apparatus of an array antenna, including a first beamforming element coupled to receive at least one of a first signal vector and a first feedback value, to form a first adaptive beam, a first least mean square (LMS) element coupled to receive at least one of a second signal vector, a known vector, and a second feedback signal to generate a first updated coefficient value, a decision element coupled to receive the first adaptive beam and to form a tentative symbol decision value, a second LMS element, coupled to receive at least one of the first signal vector, the tentative symbol decision value, the known vector, the second signal vector and a third feedback signal to generate a second updated coefficient value, and a second beamforming element coupled to receive the second updated coefficient value and the first signal vector to generate a final symbol decision value as a second adaptive beam, wherein the first feedback signal is the first updated coefficient value.
To further achieve the above-described objects of the present invention in a whole or in parts, there is provided an adaptive beamforming method including the steps of: (a) obtaining a tentative symbol decision value by processing a traffic signal vector, (b) obtaining a final updated coefficient value by performing a least mean square (LMS) algorithm at least once with respect to the tentative symbol decision value and a pilot signal vector on the assumption that the tentative symbol decision value is a transmitted symbol value, and (c) forming an adaptive beam which corresponds to a final symbol decision value using the traffic signal vector and the final updated coefficient value.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.